1. Field of the Invention
The present invention relates to a driver for an induction motor which drives the induction motor in a variable speed manner, and a method of driving an induction motor, and more particularly to a driver for an induction motor which is suitable for the case where a crane apparatus or the like with which loads are lifted is driven, and a method of driving an induction motor.
2. Description of the Related Art
When loads are lifted at an acceleration α with a lifting and lowering apparatus such as a crane, as shown in Expression (1), a necessary torque τm depends on the acceleration α, and a weight m·g of a load. Note that, it is assumed in Expression (1) that a load is lifted with a pulley having a radius r.M·α·r=J·dω/dt=τm−m·g·r  (1)where J is a total inertia moment in a system including a motor and a mechanical system (having the pulley and the like), ω is a rotational speed of the motor, m is a mass of the load, and g is a gravitational acceleration.
Here, a maximum value of the torque τm depends on a motor capacity. Thus, when a value of m·g·r in Expression (1) is large, the acceleration a in Expression (1) becomes negative or zero. As a result, it is impossible to lift the load. On the other hand, even when the acceleration a is positive, if a rate of change in speed expressed by dω*/dt is larger than the value (maximum value of torque τm−M·g·r)/J, the motor cannot follow a command, and thus the lifting fails.
Conventional vector control for an induction motor, for example, is disclosed in Japanese Patent No. 3351244. An outline of conventional speed sensorless vector control for an induction motor will now be described. When a primary frequency is given in accordance with an activation command, an exciting current command is arithmetically operated. When the induction motor is activated, a speed estimate value is arithmetically operated based on a current command, a voltage command, and the like. A torque current command is arithmetically operated so that the resulting speed estimate value coincides with the given speed command. Also, a d-axis current control system and a q-axis current control system are provided, and a d-axis voltage and a q-axis voltage are corrected so as to coincide with corresponding command values. A primary frequency command for a power converter is arithmetically operated based on the speed command, the speed estimate value, the current commands and the like. A d-axis voltage command and a q-axis voltage command are arithmetically operated by using the primary frequency command, the various kinds of current commands, and outputs from current control sections, and are converted into three-phase A.C. voltage commands by using arithmetically operated phases. Also, the power converter generates three-phase A.C. voltages in accordance with the resulting three-phase A.C. voltage commands, and the three-phase A.C. voltages thus generated are supplied to the induction motor.
In the conventional vector control, the currents and the voltages are controlled in the manner as described above so that an actual speed follows a predetermined speed command.
The lifting and lowering apparatus such as the crane operates to lift loads having a certain weight from a place, lower the loads to put them at another place, and lift other loads having a different weight in some cases. Thus, there is the possibility that the magnitude of the load largely differs every time. When the acceleration rate is always held constant, if the weight (m·g) is large, as can be seen from Expression (1), there is the case where the acceleration cannot be made. When the loads cannot be lifted, for example, there is expected a method in which after an alarm is generated to urge a worker to reduce the loads, and the worker lightens the load, the loads are lifted. In this case, however, the worker is forced to be interrupted, and thus the work efficiency is poor. When the maximum load can be supposed, there is no problem even in the case of an acceleration rate that has been initially set. However, with the conventional control method, it is impossible to cope with the case where the magnitude of the load differs every time, and thus the unsupposed load is applied to the lift.